The invention relates to line drivers for supplying symmetrical output signals to a two-wire communication bus. The communication bus consists of two wires, usually a twisted pair. If the signals on both wires of the twisted pair are exactly symmetrical, the electromagnetic fields radiated by both wires cancel, one another and there is no electromagnetic emission. Commonly, one tries to design the individual drivers for both bus wires to generate signals with identical slopes. For that purpose the drivers should start at exactly the same time since any difference in timing results in severe electromagnetic emission. This design goal is difficult to implement. The drivers for both bus lines may have different speed, for the positive and negative slopes of the output signals, for example because transistors of different conductivity type are used for the positive and negative slopes.
It is an object of the invention to provide a line driver which supplies highly symmetrical output signals to a two-wire communication bus, even at high speed.
To this end the invention provides a line driver as claimed in claim 1. Due to the propagation delay of the inverters the output terminals of all inverters down the chain change polarity one after the other. This results in a series of small voltage increases on the first wire, which voltage increases are linked to small voltage decreases on the second wire. A next small voltage step on the first wire only happens after the previous step on the second wire has finished.
In a preferred embodiment the delay function and the drive function of the line driver are separated by inserting additional inverters between the output terminals of the inverters and the resistors.
The inverters preferably have a PMOS transistor and a NMOS transistor connected in series between supply lines of a supply voltage source. Thus, each inverter of the chain is simple and the performance of the line driver is independent of the characteristics of the individual PMOS and NMOS transistors.
The line driver according to the invention can be used, for example, in Controller Area Network (CAN) bus systems in cars, provided that diodes are inserted in the connections to the bus wires to enable multiple line drivers to be used on the same CAN bus.